1. Field of the Invention
The present invention relates generally to a supply voltage generator for supplying a predetermined voltage to the internal circuits of a semiconductor device, and a semiconductor device equipped with the supply voltage generator.
2. Description of the Related Art
Some recent semiconductor devices have internal circuits which have been designed to consume less power and/or requiring various levels, such as a high voltage level and a negative voltage level, which are incorporated in a supply voltage generator. The supply voltage generator generates a desired voltage from the supply voltage applied to its external terminals, and supplies the generated voltage to the internal circuits. In such a semiconductor device, for example, when voltage application to the supply voltage generator becomes unnecessary for the purpose of reducing the consumed power, the supply voltage generator is designed so as to stop its operation.
FIG. 1 shows one example of a semiconductor device incorporating a conventional supply voltage generator. Provided on a chip 1 are an oscillation circuit 2, a booster circuit 3 and an internal circuit 4. The oscillation circuit 2 and the booster circuit 3 constitute the supply voltage generator.
The oscillation circuit 2 is connected via a pair of external terminals 11A and 11B to a crystal oscillator 5. From a reference frequency signal output from the crystal oscillator 5, the oscillation circuit 2 generates a desired oscillation output signal SG1 as shown in FIG. 2. The oscillation output signal SG1 is supplied to the booster circuit 3 and the internal circuit 4.
The booster circuit 3 is a known circuit which comprises capacitors and switching elements (neither shown). The booster circuit 3 is supplied with power from a power supply V.sub.cc through an external terminal 12 on the chip 1, and generates an output voltage V1 higher than the supply voltage V.sub.cc (see FIG. 2), in response to the oscillation output signal SG1. The output voltage V1 is set in accordance with the frequency of the oscillation output signal SG1. In response to the beginning of the oscillation of the signal SG1, the booster circuit 3 boosts the output voltage V1 to a predetermined level. The voltage to the internal circuit 4 is supplied by the supply voltage V.sub.cc through the output voltage V1, and executes a predetermined operation in response to the signal SG1.
When supply of the high-level output voltage V1 becomes unnecessary, the oscillation of the output signal SG1 from the oscillation circuit 2 is stopped and the booster circuit 3 stops the boosting operation in order to reduce the consumed power of the semiconductor device. As a result, the output voltage V1 drops to reach (or approach) the level of the supply voltage V.sub.cc in accordance with the gate capacitance or the junction capacitance of the transistors constituting a buffer circuit in the booster circuit 3.
When the output voltage V1 drops due to the termination of the oscillation signal SG1, noise N1 occurs on the output voltage V1 as shown in FIG. 2. When the output voltage V1 falls to the level of the supply voltage V.sub.cc, noise N2 likewise appears on the output voltage V1. The presence of noise N1 and N2 is undesirable since they may cause the internal circuit 4 to malfunction.
To prevent malfunction caused by noises N1 and N2, various measures have been taken. One measure is to increase the distance between the booster circuit 3 and the internal circuit 4 and increase the length of the interconnection between both circuits. Another is to provide a noise removing diode between both circuits 3 and 4.
Increasing the distance between the booster circuit 3 and the internal circuit 4 however results in a larger chip area. The noise removing effect that is provided by the intervention of a noise removing diode between the booster circuit 3 and the internal circuit 4 depends on the characteristics of that diode and the transistors constituting the booster circuit 3. The noise removing diode alone could not therefore prevent the internal circuit from malfunctioning from the noises N1 and N2.